This invention relates to an improved method for the removal of ultra-fine particulates from an aqueous suspension. This method can be used for the separation and recovery of particulates and colloids from high-ionic strength sludges and slurries. In addition, it can provide an alternative route for the production of a highly stable glass waste form.
The separation of fine particles from aqueous process streams remains one of the most difficult and expensive unit operations in the treatment of process streams, including highly radioactive wastes. The current baseline technology for the treatment of high level radioactive wastes includes the use of cross-flow nanofiltration to separate particulates and colloids from water and dissolved salts. It is very difficult to achieve the degree of particulate separation needed to produce a high-level waste concentrate and convert the bulk of the waste to a nontransuranic (non-TRU) Class A low-level waste. Membrane filtration processes are susceptible to fouling. To prevent membrane blockage, the filtration units must be regularly back flushed with cleaning solutions which become a secondary waste that must be treated and disposed of. In cross-flow filtration processes, final solids concentrations are generally limited to 15-20 wt % so that the slurry can be pumped through the membrane unit. The Theological properties of slurries containing high concentrations of salts, the tendency to form a gel, may put further limitations on the final solids concentrations achievable. Thus, large amounts of water and soluble salts remain in the concentrate after filtration.
In response to the problems associated with nanofiltration, this invention provides a method of using a sol-gel process for the removal of particulates in the treatment of processes or waste streams. Specifically, the sol-gel process is designed to separate and recover particulates, including submicron colloids, from caustic sludges and slurries at a level of efficiency sufficient to achieve non-TRU Class A low-level waste status for the bulk of the waste feed. In the alternative, the sol-gel process provides a convient route to the production of a highly stable ceramic waste form with high waste loading.
In general, the approach involves the addition of alkali silicate and an organic gelling agent directly to the caustic waste stream to immobilize particulates that range in size from macro sizes to nano-scale colloids. The particulates and colloids become immobilized within a porous, three-dimensional silica, SiO.sub.2, network. Within minutes of formation, the water and soluble salts begin to spontaneously exude from the gel while the gel simultaneously contracts, syneresis. Quantitative recovery of particulates, including colloids, is achieved by physical entrapment within the SiO.sub.2 network. Water soluble salts, originally present in the waste stream, are recovered in a separate aqueous stream. Salt recoveries from the monolith during syneresis can be in excess of 90%, and can be improved to greater than 99% by secondary washing, with a capability of recovering greater than 99.999% of all particulates, including colloids. The process is more efficient than membrane filtration and produces a rock-hard silica monolith that can be used as a feedstock to a glass melter or consolidated to near theoretical density by sintering. Because a dense waste form can be generated through a sintering process rather than through a glass melt, waste loading is not solubility limited and levels as high as 40 to 45 wt % are achievable.
It is the object of this invention to provide a method for separating ultra-fine particulates from aqueous suspensions using a sol-gel process.
It is a further object of this invention to teach the sintering of the porous gel to form a nonporous monolith by sintering at approximately 650.degree. C.
It is another object of this invention to provide a method for improving the leach resistance of the monolith by first reducing its alkali content through a mild acid wash followed by sintering to a highly dense monolith by sintering at approximately 1200 to 1400.degree. C.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.